Such a piezo actuator, also referred to as a piezoelectric actuator, is usually constructed from a plurality of piezo elements which are arranged to form a stack-shaped actuator body. Each piezo element consists of a piezoceramic layer which is provided with a metal electrode on either side. When a voltage is applied to these electrodes, the piezoceramic layer responds with a lattice distortion. As a result, the piezo element, and thus the actuator body, expands and contracts in a direction which is determined by the arrangement of the piezoceramic layer and the electrodes of a piezo element. Depending on a degree of expansion and contraction, a useful change in a dimension of a stack-shaped actuator body is produced.
When a voltage of suitable polarity is applied to a piezoceramic disc, the layer thickness thereof increases slightly. In order to achieve adjustment distances which are useful in practice, such discs are stacked and thus operated mechanically in series. The maximum adjustment distance is proportional to the overall length of the stack forming the actuator body, and the load capacity is approximately proportional to the cross-section of said stack.
The actuator body is often arranged in a steel housing so as to obtain a mechanical bias for achieving relatively high tensile forces. In principle, ceramic actuator bodies should be exposed to no or only low tensile forces, and specifically at most 5% of the compressive load capacity. The most common measure for increasing the tensile load capacity of the system is therefore that of providing a piezo actuator with a mechanical bias.
When using piezo actuators as power generators, the useful force depends, as with other actuators, on the rigidity of the fixing. For the maximum drive voltage and an infinitely rigid clamping or fixing of the piezo actuator, the maximum force which can be produced is what is known as a blocking force. It is the force required to push a completely extended piezo actuator back into its starting position.
In addition, uses for a piezo actuator as a piezo injector are also known. In a piezo injector, the piezo actuator is connected to a nozzle needle such that a nozzle opening is freed by applying a voltage to the piezo elements.
U.S. Pat. No. 6,274,967 B1 discloses a piezo actuator in multilayer construction which is provided with a biasing device for introducing force into the piezoelectric layers. By means of the biasing device, a uniaxial compressive stress is applied to the piezoelectric layers in the stack direction of the piezo actuator.
DE 196 50 900 A1 relates to a piezo actuator which is provided for example for actuating injection valves on internal combustion engines in motor vehicles and is protected from tensile stresses in that resilient biasing elements are provided for the piezoelectric body of the piezo actuator and place the piezoelectric body under compressive bias. For this purpose, the piezoelectric body has a resilient chucking means which braces the end faces against one another under compressive bias, in that for example coupling elements or end plates are interconnected by resilient tensioning bands made of round or flat material or via a tubular bellows formed in the manner of a tension spring. For example, a spring sleeve having a substantially cylindrical shape and a lower region which undulates in a bellows-like manner is used for this purpose, the undulations being under high tensile stress such that the piezoelectric body is subjected to a corresponding compressive bias.
DE 10 2005 028 970 A1 relates to a piezo actuator which, when a voltage is applied, exhibits a particular expansion response depending on this voltage. In order to achieve an increased stroke response, the piezo actuator has at least one piezoelectric layer having at least one arch, which is arranged between two opposing electrode layers for producing an electric field in the piezoelectric layer, and a biasing device, by means of which a mechanical tension can be set in the piezoelectric layer via the at least one arch, such that the mechanical tension promotes an expansion response on the part of the piezo actuator when an electric field is produced in the biased piezoelectric layer.
The piezo actuator described in DE 199 28 185 A1 advantageously comprises at least one piezo element which is capable of applying a tensile or compressive force to an actuating element. The piezo element is held between a base plate and a spring plate which also rests against a housing via a biasing spring and guides the actuating element. In another embodiment, two piezo elements are arranged symmetrically about a tension rod which represents the actuating element and surrounded by an intermediate layer in the housing of the piezo actuator. In this case, the piezo elements are held between a support plate connected to the tension rod and a fixing edge in the housing. The support plate rests against the housing via a spring for biasing.
DE 196 53 555 A1 discloses a piezo injector in which the piezo elements are biased by means of a bellows towards the foot part of the piezo actuator and are guided laterally in order to achieve a compact and short design. A laterally spring-loaded and particularly low-friction mounting of the piezo elements is achieved by means of the bellows. The bellows is made of steel or plastics material having a corresponding resilience and tightness.
DE 101 39 871 A1 discloses a piezo injector comprising a piezo actuator which acts on a valve member via a hydraulic pressure intensifier, in which the piezo elements of the piezo actuator are arranged in a sleeve provided with a corrugated bellows. The piezo elements are biased towards the foot part of the piezo actuator by means of a plate spring which rests against the head part of the piezo actuator and is supported on a guide body.
DE 100 26 635 A1 also shows a piezoelectric actuator. In the piezoelectric actuator the piezo elements are arranged one on top of the other in a stack direction to form a monolithic actuator body. A plurality of electrode layers, which are known as internal electrodes, and a plurality of piezoelectric layers made of a piezoceramic material (piezoceramic layers) are stacked alternately one on top of the other and sintered jointly to the monolithic actuator body.
DE 199 30 585 A1 discloses a piezo actuator which comprises a contact lug for electrically contacting an electrode of an actuator body. Owing to expansion and contraction of the actuator body, a mechanical stress occurs in the contact lug, which stress is minimised in that the contact seat comprises a deformation material in the form of a wire mesh for adapting the dimension to a degree of the expansion and contraction. The underlying idea is to reduce this mechanical stress in the contact lug.
An electrical component having a tension-relieved and compression-relieved soldered joint can be found in DE 38 05 121 A1.
DE 10 2004 005 943 B4 relates to a piezo actuator which comprises at least two electrode layers arranged one on top of the other and at least one piezoelectric layer arranged between the electrode layers, as well as at least one electrically conductive wire. The wire is designed such that no stress peaks, which can cause the wire to break, occur in the wire as a result of the mechanical load, in that the stresses occurring in the wire over the length of the wire are matched to one another. For example, a tensile stress can be converted into a shear stress owing to the form of the wire. As a result, a more homogeneous, approximately uniform tensile stress occurs along the wire. In one configuration the wire has a mechanical bias, for example a tensile stress, which counteracts the mechanical stress occurring during operation of the electrical component.
DE 10 2006 032 995 A1 relates to an electromechanical motor, in particular a piezoelectric ring motor, in which biasing elements having increased carrying force are used instead of the hollow springs usually used in such ring motors. The biasing elements extend parallel to the effective direction of a piezoelectric multilayer actuator. For example, the biasing element is formed by a spring band or a spring wire.
A drawback of the piezo actuators known from the prior art is the additional space requirement associated with the tensioning means, in particular with the support frame construction required in this regard as an abutment for the tensioning element. In addition, the tensioning means and the support frame increase the complexity of production, and this leads to increased production costs overall. Moreover, a high rigidity of the tensioning element leads to an undesirable stroke loss.